In present data processing systems, use is more and more frequently being made of magnetic disc memories because of their storage capacity and the relatively short time required for magnetic read/write heads to reach information anywhere on the discs from the moment the processing system supplies the heads with an instruction to find the information.
Magnetic discs carry information on concentric circular recording tracks having a width which does not exceed a few hundredths of a millimeter and which are formed on both faces of the disc. The tracks are identified by allotting them a serial number j (j being a positive integer) between 0 and N-1, where N is the total number of recording tracks. To enable information to be read and written, the magnetic heads are arranged on both sides of a disc a few microns away from it. To minimize the time to reach given information, it is necessary for the heads to move from a first track to a second track in the shortest possible time and to be positioned over the second track.
There are known shifting and positioning arrangements which meet these requirements. Certain of the known arrangements, known as bang-bang arrangements, use a voice-coil type of an electrodynamic motor, having a winding which moves linearly within a permanent magnet of cylindrical shape. The motor winding is mechanically connected to a carriage which carries the magnetic heads. This carriage includes two electromechanical transducers, one for position to determine the number of the track above which the heads are situated at any given time, and the other to determine the speed of the carriage at any given time.
The device for shifting and positioning the heads is moved with an acceleration phase and a deceleration phase, respectively termed the first and second bangs. During the acceleration phase, a constant current of a first polarity (positive, for instance) is applied to the motor winding, whereby the speed vs. time characteristic of the carriage (and thus of the heads) is similar to a rising linear function of the time that the carriage has been moving. A curve representing carriage speed as a function of carriage position at any given time is an ascending parabolic arc since speed increases as a function of position.
During the second, deceleration, phase of the movement, a reverse polarity current (negative, for instance) is applied to the motor. The carriage speed is then a falling linear function of time and the curve representing carriage speed as a function of carriage position is a parabolic arc, since speed decreases as a function of position. In order to stop the heads over the selected tracks, the carriage must have a low speed and the carriage must be close to the selected track at the end of the second phase.
It is the normal, present practice for the shifting and positioning devices to operate in a free state during the first phase, i.e., without being under control, whereas during the second phase these devices are servo-controlled with the speed characteristic of the carriage as a function of both time and space being as close as possible to what it would be in the free state. Servo-control means, including analogue and logic circuits, apply a constant current to the motor winding during the first phase of the carriage movement. During the second phase, the servo-control means responds to speed and position transducers to supply the winding with a current having a level which is a function of the remaining distance to be travelled and the actual speed of the carriage at any given time, so that the speed of the heads is zero when they arrive above the selected track.
It will be recalled that analogue circuits emit analogue signals having voltages that vary continuously between values of +V and -V, while logic circuits emit binary signals which assume only two values, either a "logic zero" or a "logic one". (In one system, "logic zero" is generally a voltage of 0 volts and "logic one" a voltage of +5 volts). In the art, each logic signal is known as a "bit".
With the current practice, if P is assumed to be the number of tracks to be traversed, the direction of the current in the coil is reversed when the head has traversed N' tracks, where N' is approximately P/2, to within a few tracks or even a few tens of tracks. Thus, to control the current in the winding it is necessary at any given time to have an accurate knowledge of the position and speed of the movable carriage. The electromechanical position and speed transducers and the circuits (chiefly analogue) associated with them thus must be extremely accurate and have the drawback of being expensive and bulky.